Hey, DNA: What is it good for?

So why do we still have bits of Neanderthal DNA hanging around after so many years? Of course it could just be random junk, but it’s more fun to think that it might be useful.

And the obvious useful thing for it to do is climate adaptation, since Neanderthals had been living in dark, cold, ice-age Europe for much longer than the newly-arrived h. Sapiens, and so might have had some adaptations to help deal with it.

Okay, so here is something related I was reading the other day, that I consider pretty interesting. So it looks like the people who live up on the Tibetan Plateau (like the Tibetans,) are really well-adapted to the altitude. No mean feat, considering that other populations who live at similar altitudes don’t seem to be as well-adjusted, despite living up there for similar lengths of time.

Well, now it appears that the Tibetans have actually been living in Tibet for waaaay longer than expected, because the original h. Sapiens who moved into Tibet intermarried with archaic hominids who had already lived there for hundreds of thousands of years, and so probably picked up their altitude adaptations from those guys.

BTW, “species” is a social construct and you probably shouldn’t bother with it here.

So what kind of useful stuff might we have picked up from Neanderthals?

First I’d like to interject that I still find declarations of “aha, we got this gene from Neanderthals and it does this!” to be speculative and prone to changing. All of the articles I’ve read tend to report the same list of stuff in a similar fashion, so I suspect they’re all workign off one or two sources, which makes everything doubly sketchy. So we’re going in here with a big “if” this is true…

Some of the results are fairly boring, like Neanderthal DNA affecting hair and skin. We already speculate that skin tone helps us deal with sunlight levels, so that’s sensible.

More interesting is the claim that Neanderthal DNA may predispose people to Type-2 Diabetes and depression.

Now why the hell would it do that? It’s probably not *just* random–after all, large stretches of DNA have little to no Neanderthal admixture at all, suggesting that genes in those spots just weren’t useful, so why would we have retained such apparently negative traits?

Maybe, like sickle cell anemia, these things actually have a positive function–at least in the right environments.

I read a fascinating theory a few years ago that Type 2 Diabetes and Seasonal Affective Disorder are actually just part of our bodies’ natural mechanisms for dealing with winter. Basically, you’re supposed to eat plants and get fat all summer long, while plants are available, and then by winter, your ability to absorb more glucose shuts down (there’s no point since the plants are all dead) and you switch over to burning ketones instead and eating an all-mammoth diet.

(Some groups, like the Inuit and Masai, historically [and may today still] survived on diets that included virtually no plants and so ran all of their cellular energy needs through the ketogenic instead of the glucose system.)

During this winter time, humans, like other animals, slowed down and semi-hibernated to save energy and because why the fuck not, it’s dark and no one has invented lightbulbs, yet.

By spring, you’ve lost a lot of weight, the plants come back, and so does your ability to use glucose.

This theory is laid out in the book Lights Out by T. S. Wiley, if you’re curious. I thought it was a really interesting book, but you might just think it’s all crank, I dunno.

Anyway, a big hole in Wiley’s plot is how we actually got this adaptation in the first place, since it’s a pretty complicated one and h. Sapiens hasn’t actually been living in places with winter for all that long. Wiley just claims that it’s a deep internal mechanism that animals have, which always struck me as kinda bunk because why would a species that evolved in Africa, from other animals in Africa, etc., probably going back for million upon millions of years, have some sort of complicated system like this still functional in its genome? A trait that is not undergoing positive selective pressure is probably going to become non-functional pretty quickly. But the theory was cool enough otherwise to ignore this bit, so I’ve kept it around.

Right, so here’s the (potential) answer: h. Sapiens didn’t have this adaptation hiding deep inside of them, Neanderthals had it. Neanderthals had been living in cold places for way, way longer than h. Sapiens, and by inter-breeding with them, we got to take advantage of a bunch of cold-weather adaptations they’d developed over that time frame–thus getting a jump-start on evolving to cope with the weather.

At any rate, if Wiley is correct, and SAD and Type-2 Diabetes are actually part of a dealing with winter complex that benefited our cold-weather ancestors, then that wold explain why these genes would have persisted over the years instead of being bred out.

An easy way to test this would be to compare rates of Type-2 Diabetes and SAD among African immigrants to Europe/other wintery latitudes, African Americans (who have a small amount of Euro admixture,) and Europeans. (Watching out, of course, for Vit D issues.) If the Euros have more SAD and Type-2 Diabetes than Africans living at the same latitude, then those would appear to be adaptations to the latitude. If the Africans have more, then my theory fails.

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Vitamin D may be the hormonal signal that makes the mechanism run – its a pretty good signal that correlates with winter. Low vitamin D could flip the switch to the ‘no carb, ketogenic get sad and stay asleep’ mode. With less and less exposure to the sun year round in modern society, and lower vitamin d levels across all populations, it could leave this switch permanently turned on. Than then can explain the increasing prevelance of diabetes. (in addition to having carbs available in the winter in modern society.)

Especially so if diabetes affects Africans with european (or other) admixture. Double whammy if they inheret the ‘neandertal SAD’ gene and would have a hard time getting enough vitamin d.